I think slaughter plants should be reopened, but more strictly regulated than they were. The only way I can think of diminishing the need is to stop breeding all together -- not just the 'backyard breeders', but the 'responsible' ones, too. Because even the people breeding top-quality horses are pumping out piles of foals, and even they have the chance of breeding undesirable horses, or even horses that they just can't sell for whatever reason. In a perfect world, every horse that hits the ground is automatically of good enough temperament and conformation to, at the very least, be some little girl's dream pony, some farmer's work horse, or some family's expensive yard decoration. But the fact of the matter is, not every horse is suited to be ridden, or was just born in the wrong place and put up for sale for more than that little girl or that farmer can afford. And the breeder certainly can't just keep every horse that they can't sell, and rescue centers can't hold on to every horse that stumbles through their gates.
In my area, there are several stories going around. I won't vouch for their accuracy, but they're real possibilities. One that I know is true, because I saw it with my own eyes, is of a local breeder -- He breeds Paints, high-quality, high-dollar, real nice horses, at least for around these parts. He hauled seven of his horses -- nothing wrong with them, mind you, he just couldn't sell them -- to an auction house and didn't sell a single one. Out in the parking lot, he refused to load them in his own trailer -- I flat out told everyone that if they wanted one, they could take it home, and whatever horses don't leave the lot, he was going to haul them up to Canada and drop them off at the slaughter houses. Only two of those horses walked off the lot in someone else's hands, and one of them came back the very next auction.
Another one my uncle tells: A friend of his was at a riding trail and he left his trailer parked in the lot. When he came back, there were two horses in his trailer and no one else around. He called the DNR to get rid of them, and they refused to take them. This guy had to call for a second trailer, because there wasn't room to put his own horses in his own trailer. My uncle got one of those horses, and I don't know what happened to the other, but the one he got is probably one of the craziest S.O.B.'s I've ever seen, and he had to have it put down because, after four trainers and three years of work, it still couldn't be so much as haltered, and had actually given me a slight concussion.
Other stories floating around -- I'm sure some of you have heard variants -- include horses being left behind in a move, or let loose on the road, dropped off in people's pastures, etc., etc.
Now, would you rather these horses slowly starve or be attacked by wild animals, or be shot at by non-horse-friendly people who want them off their property, or other equally painful deaths because they were abandoned, or suffer a quick death at the hands of a regulated slaughterhouse?
Everyone says that slaughtered horses could be some little girl's dream, but not every little girl can afford a horse, and not every little girl is suited to have one. There's also the fact that, not every little girl is going to grow up into a horse-loving woman or teenager. It's like puppies and Easter bunnies -- the novel of owning a pony wears off, no one wants to take care of it after awhile, and it either gets sold, or is left neglected. I work at the only tack shop in town, and I've seen all sorts of parents come in lately, with good intentions, I'm sure, telling us "We just bought a horse for our little Jane. What do we need?" So many of them are entirely unprepared for the responsibility. So many families that previously wanted to buy a horse for their kids but couldn't afford the initial buying fee are now running out and snatching up a matching pair of pinto-ponies for a total of $50, and not thinking about what it costs to raise those horses.
If this were a perfect world where every horse got the care it needed, and everyone had enough money to properly raise their families and their horses together, I'd be against slaughter all the way; But, in a perfect world like that, there would still be slaughter -- there would still be that market for meat, just like there is for beef and pork and chicken.